Devices useful for the controlled release of active ingredients are known and generally comprise a membrane which controls the rate of release and some type of carrier in which the active ingredient is either soluble or partially soluble and thus permeable to the passage of the drug. See, for example, U.S. Pat. Nos. 4,161,283, 3,993,072 and 3,926,188. The principal problems with all of the prior art, including the dispensers of the patents mentioned, are that they cannot maintain a zero-order rate of release over the entire life of the dispensation while at the same time dispensing virtually all of the active ingredient.
A zero-order release rate is an essentially constant rate of release of active ingredient that is independent of the amount of the active ingredient and its vapor pressure and is often of critical importance in the delivery of a given active ingredient; for example, the delivery of pheromones as insect attractants is not effective unless the rate of delivery is within certain very narrow limits. To the extent all of the active ingredient is not dispensed, there is a substantial waste and consequent expense inasmuch as the active ingredient is often quite expensive and the remainder simply must be thrown away with the dispenser.
What is needed therefore, is a controlled-release dispenser which is capable of delivering essentially all of an active ingredient at a selected zero-order rate of release over a broad range of release rates which is inexpensive and easy to fabricate.
These and other objects are accomplished by the controlled-release dispenser of the present invention which is summarized and particularly described below.